Processes have been developed for producing formed articles such as engine components and superconductive composites from powdered metals or powdered ceramics. One conventional process involves the compounding of fine ceramic or metal particles with a binder and a solvent carrier to form a self-supporting shaped article, referred to as a "green body." The green body is then heated in a controlled atmosphere to pyrolyze the solvent, binder, and any other additives present, and to sinter the particles together to form a highly dense, homogeneous fused product.
Typical binding agents used in such processes are paraffin waxes and polyethylene plastic, which are used with hydrocarbon-solvents, such as heptane, hexane, or decane. The paraffin wax or polyethylene binder typically comprises between 10 and 30 weight percent of the green body. When the presence of the hydrocarbon solvent is factored in, typically less than 40 percent of the weight of the green body is attributed to the ceramic or metal particles. The result is a green body that is prone to cracking and shrinking during pyrolysis. The finished product has a low density and may contain voids with attendant weak mechanical characteristics. As a result, the rate of rejection of parts produced by such methods is extremely high, on the order of 90%. Additionally, pyrolysis of the wax or polyethylene and the hydrocarbon solvents may result in the production of toxic by-products.
Another conventional type of method for producing articles involves the suspension of colloidal ceramic or metal particles in a liquid carrier containing a particle dispersant agent and a gelling agent. The suspension must have a low viscosity to enable introduction of the suspension into a mold. After introduction into the mold the suspension is caused to gel, forming a self-supporting article that is then pyrolyzed and sintered.
A long standing difficulty in working with submicron-sized ceramic particles in suspension processes is the tendency of the particles to aggregate within the carrier due to van der Waals attractive forces between the particles. Such aggregation or agglomeration of the suspension creates larger effective particle sizes and leaves undesired voids in the finished product, resulting in cracks or weak spots in the finished product. To avoid such defects, it is desirable to uniformly disperse the particles in the liquid carrier to form a nonagglomerated, stable suspension of densely packed particles, resulting in a high density, high strength finished product.
To this end, conventional dispersant systems have been developed that use polyelectrolyte dispersants to coat the particles, creating electrosteric interactions between the particles that counteract the attractive forces to disperse the particles. For example, dextran sulfate has been demonstrated as being a suitable dispersant for producing stable, highly loaded aqueous suspensions. G. L. Graff et al., Processing of Ceramic Suspensions With Biopolymers (ACS International Conf. Colloid and Surface Science, Seattle, Poster Presentation, 1989). Dextran sulfate systems overcome many of the problems of polyethylene and paraffin wax systems due to the relatively low volume percentage of dispersant required. However, dextran sulfate is not an ideal dispersant due to the incomplete removal of the sulfate functional groups during pyrolysis, resulting in a contaminated final product.
Other conventional polyelectrolyte dispersant systems have been developed that utilize synthetic polymer dispersants to produce aqueous suspensions of ceramics and metals. Examples of such synthetic polymer dispersant systems are offered by U.S. Pat. Nos. 4,816,182 and 4,904,411, both issued to Novich et al. Novich '182 discloses the use of acrylic acid-based polymers in a water carrier, and triethanolamine and carboxylic acid in an alcohol carrier, as suitable dispersants for ceramic and metallic colloidal particles to create highly solid-loaded pourable suspensions. Additionally, Novich '411 discloses the use of polyethylene imine-based polyelectrolytes as suitable dispersant agents. The conventional synthetic polymer-based dispersant systems disclosed by Novich '182 and '411 enable the production of high density ceramic and metallic parts, but have the drawback of toxicity of the polymer dispersants, the monomer precursors of the polymers, and the by-products of the pyrolysis process. The alcohol carrier-based nonpolymeric dispersant systems disclosed by Novich '182 are also unsuitable due to the cost and toxicity of the alcohol carrier.
Another example of a conventional synthetic polymer dispersant system is disclosed by U.S. Pat. No. 4,734,237 to Fanelli et al., in which a metallic or ceramic powder mixture is used in injection molding of high density parts. The mixture includes between 50 and 90% by weight ceramic or metal powder, a dispersant, a gel-forming material, and a solvent for the gel-forming material, typically water or alcohol. The gel-forming materials disclosed are agar and agar derivatives such as agarose and agaroids. Various synthetic dispersants are disclosed for use in the mixture, including Darvan C.TM., a vinylidine cyanide vinyl acetate copolymer. Several other dispersants, such as gum arabic, are stated to be unsuitable for use in the mixture due to the deleterious effect on the gel strength of the agar-type gel-forming material.
In addition to the toxicity of the dispersants used in synthetic polymer dispersant systems such as those disclosed by Fanelli et al., a significant limitation of such systems is the common phase incompatibility of the synthetic dispersants with biologically produced gelling agents. The synthetic dispersants, essentially derivatives of petroleum, are not miscible with the natural gelling agents, and tend to separate out within the mixture. Such systems have not been subject to widespread commercial adaptation because of this problem.